1. Field of the Invention
The present invention relates to sound image display techniques for visually displaying a sound image formed by a multi-channel audio signal in a reproduced sound field, and more particularly to a sound image display method and apparatus which is particularly suitable for monitoring localization, spacial impression and phase of a sound image in the production of audio products utilizing surround techniques.
2. Background of the Invention
A prior art sound image display technique is described in an article entitled "Development of Surround Audio Monitor" in a magazine called "Hoso-gijutsu (Broadcasting Techniques)" (1993, vol. 46, No. 8, pp. 77-81). A surround audio monitor proposed in the article is adapted to monitor the localization, spacial impression and phase of a sound image formed by a surround audio signal such as a Dolby surround audio signal as well as 3-1 configuration four-channel stereo and 3-2 configuration five-channel stereo signals by displaying the sound image on an X-Y scope. A commercially available video analyzer (Type VM700A) manufactured by Sony Tektronix Co. functions to display a Lissajous waveform of a two-channel stereo audio signal on the screen of an X-Y scope. The video analyzer is provided with an automatic gain control (AGC) function for performing the automatic gain control on X-axis and Y-axis components inputted to the X-Y scope on the basis of the higher level one of the X-axis and Y-axis components when a LissaJous waveform is displayed.
The above-mentioned surround audio monitor displays sound images based on four-channel/five-channel surround audio signals in an X-Y coordinate system. For this purpose, X-axis and Y-axis components of a sound image are calculated as products of sound pressure P and particle velocities (U.sub.X, U.sub.Y). It will therefore be understood that the calculations of the X-axis and Y-axis components of a sound image present an input/output relationship of a square characteristic. Specifically, if an input is decreased to one half of a previous value, an output will be decreased to one quarter. Conversely, an input increased by a factor of two will result in an output which is increased by a factor of four. Since the sound image is displayed on a limited display space of an X-Y scope, if an input signal is small, the observer will have difficulty in tracing its sound image trajectory displayed on the X-Y scope (the trajectory will hereinafter be simply called the sound image waveform). On the other hand, if an input signal is large, the sound image waveform is scaled out, so that the entire waveform can not readily be recognized by the observer.
Also, with the above-mentioned method of displaying a Lissajous waveform on a video analyzer, the monitoring of the localization and spacial impression of a sound image for a surround audio signal is difficult.